bluesunco Can I install GrapheneOS without a SIM?
You for sure can install (and use, for the matter) GOS without any SIM inserted.
bluesunco Can I use Vivaldi for the Web Install?
I would suggest to change browser. In the official guide, in the prerequisite section they specify which browsers are officially supported:
Officially supported browsers for the web install method:
Chromium (outside Ubuntu, since they ship a broken Snap package without working WebUSB)
Vanadium (GrapheneOS)
Google Chrome
Microsoft Edge
Brave (with Brave Shields disabled, since it caps storage usage at a low value to avoid fingerprinting available storage)
You can try with Vivaldi, but the chances to get help if you encounter some issues are smaller.
If you can, stick with the prerequisites and you should be fine :)